Aim: The study is designed to measure the Long-term effects of oral contraceptive use on cancer incidence rates and mortality with particular focus on a possible increased risk for breast cancer in young middle-aged women. An increased risk for breast cancer would change the overall positive health effects of oral contraceptive use in a negative direction. Design: The study is a prospective cohort study consisting of 60,000 women drawn at random from the Central Person Register in the Central Bureau of Statistics in Norway. The women will be aged between 34 and 48 years at start of follow-up. Information on oral contraceptive use and other risk factors will be gathered by postal questionnaires. Complete follow-up till ten years after recruitment will be collected by linkage to the files of the Cancer Registry of Norway and the Register of Deaths in the Central Bureau of Statistics by use of the unique identification number given all inhabitants of Norway. Statistical analyses will be based on a case-control sampling in the cohort with four controls per case. The use of a nested cae-control design was chosen to reduce the cost of data punching and data analyses. The study is designed to find an increased risk for breast cancer among oral contraceptive users in the order of 50 percent. The proposed design will mostly eliminate selection bias and recall bias, partly detection bias, but not non-differential misclassification bias. The study will give unique opportunities to estimate population attributable risk due to the study base sampled from the total population.